OTTAWA - Prime Minister Stephen Harper urged NDP Leader Tom Mulcair to "do the right thing" and reimburse taxpayers roughly $3 million that may have been spent illegally.

"I have noted that the NDP leader has not repaid the taxpayers of Canada for $3 million used for parliamentary offices (outside) Ottawa, contrary to the rules of the House of Commons," Harper said in French during question period Wednesday. "It is time for the NDP leader to do the right thing."

The House of Commons board of internal economy is investigating the official Opposition after receiving complaints about NDP satellite offices in Quebec and Saskatchewan being staffed with employees on the House of Commons payroll.

House rules dictate that a political party must pay for partisan work itself, not with House of Commons resources.

The NDP said its Quebec offices were being used to service its 57 MPs in that province and explained its satellite office in Saskatchewan as a means of conducting outreach.

The board issued a cease and desist order to the NDP and adopted a provisional amendment to the bylaws governing MPs in the meantime.

"No employee of a member of House officer may have as their regular place of work any space in premises owned, leased or under the effective control of a political party," the provisional amendment reads.

That amendment is set to expire in 2015, which the NDP says proves the investigation is a partisan attack. NDP sources also say the party had cleared plans for these satellite offices with the clerk of the House of Commons, who told them to "go ahead."

The NDP will have to shut its satellite offices Monday.

If the board - which operates in camera and includes members from all parties - finds the NDP did breach House rules, the party could be on the hook to repay roughly $3 million.

The NDP has called the investigation a partisan attack and said the board of internal economy should be disbanded.

"We've long been saying that we must remove the board of internal economy's powers and create an independent body to investigate MPs' expenses," NDP House leader Peter Julian said in a statement issued earlier this week.

Conservative MP Randy Hoback - who complained to the board about the NDP's Saskatchewan office - said he doesn't believe the party was doing outreach work, but rather that they're trying to increase its electoral chances in Saskatoon's new ridings.

Echoing Harper, Hoback said taxpayers deserve to be repaid.

"The fact is they've been caught," he told QMI Agency. "The reality is they owe the taxpayers $3 million and the NDP is scratching its head on how to pay that back."

Hoback said there are already 14 constituency offices in Saskatchewan providing services to constituents there.

"For them to come in and duplicate that would be a waste of taxpayer money," he said. "Now, if they're not duplicating that then they're not doing the functions of a constituency office and thus that means they're political, which means they should pay for the work, not taxpayers."